1. Field of the Invention
This invention relates to an electrode and a method for maintaining the dimensions of the electrode and, more particularly, this invention relates to a dimensionally stable electrolysis anode and a method for maintaining the anode during electrolysis.
2. Background of the Invention
Using electrolysis to separate aluminum metal from pre-processed ore has been well known, particularly after implementation of the Hall-Heroult process. In the Hall-Heroult process, Aluminum derived from predigested Bauxite feedstock is generally produced by electrolytic reduction. The aluminum feed for that process is Al.sub.2 O.sub.3 dissolved in a bath of molten 3NaF.AlF.sub.3 (cryolite) and AlF.sub.3 at a temperature near 960.degree. C. Aluminum ions are reduced to aluminum metal at the cathode, while at the carbon anode, the anion ("X") is combined with carbon to form C-X, typically oxides of carbon. In situations wherein the electrolytic bath contains low oxygen levels, side reactions of the anode also generate perfluorocarbons (CF.sub.4 and C.sub.2 F.sub.6).
The overall electrolytic, aluminum production process, wherein carbon anodes are consumed, can be represented by Equation 1, below: EQU 2Al.sub.2 O.sub.3 +3C.fwdarw.4Al+3CO.sub.2 Equation 1
Aside from the generation of carbon by-products associated with the use of carbon anodes, the very manufacture of carbon anodes also causes emissions of polynuclear hydrocarbons, volatile organic compounds, HF, SO.sub.x, COS, NO.sub.x, CO and CO.sub.2.
Carbon anode losses of one-half pound for every pound of aluminum produced are not uncommon. As a result of the above-stated anode-consumption phenomenon, carbon anodes must be continually replaced to facilitate continuous operation of the electrolytic process.
However, prior to complete replacement, the degrading anodes must be continually repositioned so that an optimal anode-cathode distance is maintained during electrolysis. Otherwise, power losses occur, leading to higher electricity requirements. That the anode continually changes shape also makes it more difficult to maintain a uniform anode current loading.
Magnesium also is generally produced by electrolytic reduction. In this instance, a chloride-based feed (MgCl.sub.2.xH.sub.2 O, where x is approximately 2) is utilized. Reduction proceeds with chlorine gas released at the anode and magnesium metal production at the cathode. In magnesium processing, concomitant oxidation of the carbon anode by the oxygen in the magnesium feed leads to further consumption of the anode.
Maintaining an optimal electrode gap is even more difficult in magnesium processing scenarios. Unlike aluminum, which forms at the bottom of an electrolyte bath, any produced magnesium travels between the electrodes and ultimately floats to the electrolyte surface. As such, the cathode and anode must be juxtaposed to each other to define a space (through which the magnesium metal travels) with the anode at the center of the bath and the cathode defining the sides of the bath. Also, magnesium processing causes heavier corrosion at the depending end of an immersed anode, compared to elsewhere on the electrode. Magnesium processing results in the anode surfaces becoming unevenly corroded and therefore not equidistant to the opposing surfaces on the cathode.
Efforts have been made to produce a dimensionally stable or non-consumable anode. Most of the research has concentrated on oxide-based ceramic anodes and cermet anodes (nickel-ferrite and copper metal dispersions). However, these efforts have generated anodes with relatively higher resistance, lower fracture toughness, nonuniform current distribution, or porosity problems.
Many pure metals have been considered as anode fabrication material alternatives. However, these pure metal electrodes experienced high corrosion rates, with oxide layers forming and flaking off.
Several refractory compounds, such as TiC, ZrB.sub.2 and MoSi.sub.2 were also briefly considered as anode material candidates. However, these materials were found to corrode during electrolysis.
U.S. Pat. No. 4,999,097 describes a metal electrode containing a protective coating. The coating comprises a material that is not substantially reduced by the metal product being formed and is not substantially reactive with the electrolyte. However, no provision exists therein for in situ reforming or maintaining the coating which becomes damaged either from mechanical agitation or thermal cycling inherent in the electrolytic process. Rather the '097 process requires that high levels of material forming the protection layer be in the electrolyte to initially form and maintain the layer. Furthermore, the protective layer constituents must be selected so as not to react with or dissolve in the electrolyte.
U.S. Pat. No. 5,510,008 describes a porous anode structure to facilitate in situ formation of an oxide protective layer. However, as with the '097 disclosure, no method for repairing or maintaining the protective layer is provided.
U.S. Pat. No. 5,185,068 describes a dissolvable anode which serves as an electrolyte constituent feed source, thereby obviating the need for an additional electrolyte feed source.
U.S. Pat. No. 5,254,232 discloses an oxide layer on an anode that is operational only if the material comprising the layer is present at saturation levels in the bulk electrolyte.
A need exists in the art for a dimensionally-stable anode that can be replenished in situ. The anode should be operable with existing electrolytic processes and existing electrolytes. The anode also should be operable in a myriad of electrolytic environs, (including electrolytes containing chlorides) and bath ratios. Lastly, the anode should be operable in advanced electrolytic cells wherein wettable cathodes are employed so as to minimize and therefore optimize electrode gap distances.